The invention pertains to a drive circuit for brushless dc motors.
Drive circuits of this kind are known from DE-OS 35 37 403 A1. These circuits exhibit a number of properties that are advantageous for the operation of a commutatorless dc motor.
Sample functions include:
low loss speed control of the motor,
temperature governed speed control of the motor,
signal output in case of excess temperature,
reduction of acoustic and EMI radiation.
One particular advantage of these circuits consists in the fact that they can be fully integrated onto a relatively small silicon module (chip). There, these circuits on their allocated silicon module, represent a very high functional density. It is therefore difficult and complicated to implement additional module functions of this type since signal functions and performance control functions will have to be present side-by-side on the same silicon chip.
For greater power or higher speed motors, however, more extensive speed control functions and control functions are required.
In addition, it is an advantage to achieve a yet higher efficiency for the motor and associated drive circuit.
Therefore, it is the task of the invention to describe an electronic circuit that will ensure an improved overall efficiency for the drive circuit and motor by means of a wider speed range and to include an expanded level of function.
The invention is suitable both for single phase motors with permanent magnetic auxiliary torque according to DE-OS 23 46 380, and also for multiphase motors or commutatorless motors without permanent magnets (reluctance motors).
The invention is based on the fact that for purposes of additional energy savings through increasing the overall efficiency of the motor and drive circuit, the following activities are be combined:
First, according to this invention it is an advantage specifically to shift in time the rising and/or falling edges of motor current pulses by means of a suitable circuit, and, of course, especially as a function of the speed of the motor. This will achieve both reliable start-up behavior and also good efficiency for the motor.
For all motors in the range of high and maximum speed, it is an advantage to shift forward the moment of connection of a motor current pulse in order to provide the necessary current maximum in a timely manner due to the finite rate of rise of a motor current pulse. Depending on the motor and speed range, shifts of the commutation timing by several ten degrees (electric) is an advantage.
In the case of motors with permanent magnetic auxiliary torque, it is particularly useful to shift the turn-off edge of a motor current pulse forward in time within the range of minimum speed (increase of the extinction angle), where the stated extinction angle can amount to as much as about 90xc2x0 (electric).
After start-up of the motor (ramp phase) within an average speed range, normally no essential shift in the moments of commutation is needed, whether for the ignition angle at the beginning of current flow or for the extinction angle at the end of a motor current pulse.
In this range, therefore, we can succeed, even without specific measures, in bringing the time rise of a stator current pulse into approximate coincidence with the shape of the induced counter-EMF of an (unconnected) stator coil, in a generally known manner.
Secondly, the first activity according to this invention, is combined with a circulated (clocked) current flow to the motor (i.e., usually the stator), and, of course, with a clock frequency preferably outside of the range of human hearing.
This measure has the advantage that by means of differing individual pulse widths in the clocked method of current flow, the effective motor current can be preset and adjusted in a simple manner without any large losses in performance due to this adjustment process, as is already generally known. In addition, in a similarly known manner, the advantage results that the effective maximum rate of rise of the motor current can be increased through simultaneous increase in the supply voltage.
It is understood that the supply voltage cannot be increased indefinitely, so that also the effective maximum rate of rise of the motor current remains limited to a finite value.
For high motor speeds it is therefore an advantage to effect a preshifting of the injection timing, even in the case of circuited or clocked motor current.
In this case, one decisive parameter, in addition to the motor speed, is the electric time constant of the motor coils: the ratio of inductivity to resistance, L/R.
In addition, according to the invention it is an advantage to modify the high frequency timing of the motor current during one commutation phase in the direction of smaller pulse duty factors, and, of course, toward the end of one commutation phase. It has proven advantageous to reduce the percentage of the pulse duty factor in two steps each of about 5% after passage of 50% and 75%, respectively, of one commutation phase. This will also avoid additional expense as is necessary in a continuous reduction of the pulse duty factor during one current flow phase, and, furthermore, the latter method has the same or better power reduction effect.
In a third aspect of the invention actions are illustrated that will ensure the provision of differing ignition or extinction angles and thus cause a preignition or advanced extinction of a motor current pulse.
Proceeding from a standard ignition with an ignition angle of zero, it is initially not possible in the range of high and maximum speed of the motor to implement in advance any commutations without a knowledge of future moments of commutation, as this is actually necessary for preignition (according to definition).
According to this invention, in order to solve this problem, the signal output for a commutation is undertaken by means of a galvanomagnetic sensor. However, in this case the sensor is intentionally placed at a location, e.g., between stator and rotor, that will cause a forwardly shifted signal output of, e.g., 5xc2x0 (electric), as compared to normal signal output. By means of a delay feature in the drive circuit, a delay in these commutation signals will also be possible. Thus it is possible to generate advance ignition angles up to a specified angular value. A delay feature that can delay the commutation signals in a variable manner is a particular advantage, so that the actual start of a commutation process can take on any value between the stated value and later phase angles or moments.
At greater speeds that can necessitate a more prominent forward shift of the ignition angle, the problem again arises of the absence of knowledge of the phase position of the rotor at the desired moment of ignition, at which the commutation process of the stator current is to begin. In addition, the problem exists that the optimum moment of commutation is being shifted constantly, i.e., depending on the speed of the rotor, it consists of another phase position or speed setting of the rotor.
However, according to the invention, in this case, a solution is possible according to the following logic:
The additionally necessary phase preshifting of the moment of ignition occurs practically only at high speeds. At these speeds, the rotational motion due to the mechanical inertia of the rotor, i.e., of the stored mechanical energy, is determined practically for several future rotations of the rotor.
Accordingly, it is possible, with a knowledge of the current rotational speed of the rotor and of the last moment of signal output of the rotational position sensor, to calculate a moment that corresponds to a desired or necessary, future moment of ignition at the current rotational speed (angular velocity) of the rotor.
In this case, according to the invention, a generally known device will be used that determines the rotational velocity of the rotor from the progress of the last determined sensor signals.
In this case, the following mathematic relation will be used:
Z=K(n)+(K(n)xe2x88x92K(nxe2x88x921)*(1+xcfx86(xcfx89)/360xc2x0 (el.))
where Z is the time of the prognosticated or extrapolated, future, next moment of ignition, while K(n) or K(nxe2x88x921) is the moment of the last or next to last output signal of the sensor.
xcfx86(xcfx89) represents the necessary speed-dependent angle of the preignition which always has a negative sign with regard to a standard commutation with ignition offset 0.
In a preferred circuit design it is provided that the forward shifting of the ignition timing increases linearly with increasing speed of the motor. This takes placed in a range between a motor-typical reference speed and the maximum permissible motor speed.
The rise in the associated characteristic curve that shows the forward shift as a function of the speed is also typical of the motor. Reference parameters of this type that are typical of the motor, will be placed in a simple manner as table values in the memory of the circuit configuration.
It is a precise method to perform the forward shifting of the ignition timing in a nonlinear manner, for example, according to a characteristic curve in the form of a hyperbola. Parameters of this type can also be stored in the memory area of a circuit device.
A compromise solution between linear and nonlinear forms of the characteristic curves according to this invention consists in approaching nonlinear regions by means of stepwise, linear regions.
For particular regions it is possible according to the invention, also to implement unsteady transitions of the characteristic curve.
The actions to shift the falling edge of a (stator) current pulse will also be carried out accordingly with a delay device to delay the rotational setting-sensor signal for the definition of the moment of commutation.
One preferred solution consists in delaying the signal of the rotary position sensor by a comparatively long time span by means of a delay device of this type in order then to suspend the current flow to the motor temporarily at the delayed moment.
This can occur, e.g., by specifying a pulse drive factor with value zero. This method will be simplified in an advantageous manner by the use of microprocessor functionsxe2x80x94likewise for the design of the delay itself.
It is also the advantage to make the stated delay of the signal of the rotational position sensor dependent, in a nonlinear manner, on the speed of the motor.
In particular, in the case of motors with permanent magnetic auxiliary torque, a delay feature with the following operation will be used according to this invention.
In the ramp range up to a speed of about 600 rpm, an extinction angle of 0xc2x0 (electric) will be retained; that is, no preshifting of the shut-off timepoint will take place. Above this speed value, an extinction angle of about 40xc2x0 (electric) will be specified which will be reduced up to a rated speed of about 3000 rpm in a continuous, preferably linear manner to the value zero.
In every case it is an advantage to place values belonging to the curve profile into a memory or in an electronically readable table.
Therefore, it is a benefit to provide a circuit design in the device for the above stated functions of the control circuit that contains a microprocessor or controller.
According to this invention this, opens up the potential to provide a number of additional functions for the control circuit, where the increased hardware and software effort in comparison to the attainable, additional benefit is small or is to be judged favorably.
If an expansion of this kind of drive circuit is to be undertaken at a later time, so that yet more functions will be added, then, as a rule, this is not particularly difficult.
Now the additional functions pertain primarily to checking and control or regulation functions that are supplied both with regard to external physical parameters, and also with regard to internal motor parameters.
For the stated external physical parameters, we are dealing, for example, with temperatures. This will be measured, e.g., by means of a temperature-dependent resistor and fed to a microprocessor that converts the different levels of resistance values into temperature values. The resistance values will be checked for plausibility. Deviations from a permissible value range will be recognized by the microprocessor and cause an alarm signal.
According to a computed temperature or another physical parameter, the microprocessor or the drive circuit determines what speed the motor is to use by means of a predefined table of by means of a specified function. This desired speed will be compared with the actually occurring speed. Deviations between the desired and actual speed will be minimized or eliminated through an appropriate change in motor current.
In this case, the drive circuit has a device, as discussed above, with which the motor current can be fed at high frequency in differing wide, individual pulses. By means of this current setting feature, therefore, a speed control is possible for the motor. Furthermore, it is also possible to limit the motor current with this device.
When the rotor of the motor is blocked, a speed control device usually tries to bring the motor back into rotation by means of maximum current.
However, this can lead to excessive motor temperature and possibly to damage to the motor.
Therefore, according to the invention, a device is provided that will determine the motor current punctually at specified times, and limit the motor current to a specified value.
For the case of a blocked rotor, a device is also provide that will cause a short-term, super-elevated motor current, whereupon rated current will be fed to the motor for a time and a check run will be initiated to determine whether this kind of start attempt has set the rotor in motion. If this is the case, then a normal motor power feed will take place as described above.
Otherwise, a current interrupt will occur, so that the motor can cool off. After a preadjustable time, a renewed start attempt will be performed in the same manner.
In addition, it is possible to perform improved alarm functions for motor operation. These functions consist, for example, in maintaining a constant coordination of motors when several motors are operating in parallel.
For this purpose, of course, communications or alarm signal features can be provided that are performed according to this invention by means of the mentioned microprocessor. Another improvement for an alarm function consists in the fact that the output alarm signals can contain supplemental information so that an easier and more reliable validation of a registered alarm is possible.
This will be set up according to the invention so that a valid alarm signal may have only one prespecified minimum or maximum size. Another solution consists in an alarm signal having at least one ac voltage signal component of predefined frequency and amplitude characteristics.
Additional designs and explanations of the invention are found in the subclaims and from the figures.